Immunotherapies are becoming attractive alternatives to chemotherapies, including immunotherapies that use adoptive transfer of genetically modified T cells to “reteach” the immune system to recognize and eliminate malignant tumor cells. Genetically modified T cells express chimeric antigen receptors, which generally consist of a signaling endodomain, a CD3-zeta transmembrane domain and an extracellular single-chain variable fragment (scFv) derived from a monoclonal antibody which gives the receptor specificity for a tumor-associated antigen on a target malignant cell. Upon binding the tumor-associated antigen via the chimeric antigen receptor, the chimeric antigen receptor expressing T cell (CAR T-cell) mounts an immune response that is cytotoxic to the malignant cell. Such therapies can circumvent chemotherapy resistance and have been shown to be active against relapsed/refractory disease, resulting in sustained remissions for some chronic lymphocytic leukemia (CLL) and acute lymphoblastic leukemia (ALL) patients. However, these therapies require further investigation and optimization, as they caused undesirable effects such as toxic lymphophenia, chronic hypogammaglobulinemia for hematological targets, fatal off-target cytolysis for solid tumor targets, persistent B cell aplasia with the use of anti-CD19 antibody expressing CAR T-cells, and, in some cases, death.
Introduction of a switch, which controls the activity of the CAR T-cell, would allow CAR T-cell activity and associated immune responses to be turned off after neoplastic cells are eliminated and would allow B cells to reproliferate. Recent preclinical studies have demonstrated that CAR T-cell systems can be controlled through an antibody-based switch, wherein the antibody binds the target cell (e.g. cancer cell), blocking the CAR T-cell from binding the target cell and “switching off” CAR-T activity. While these systems conceptually allow for switchable targeting of tumors using CAR T-cells, they may suffer from a series of limitations. Non-specific labeling of antibodies using cysteines or lysines produces heterogeneous products which includes variants that may be non-functional, have unpredictable pharmacokinetics and/or immunogenicity, and that may be difficult to optimize.